Street-Lynx Rear Suspension

Chrysler enthusiasts have lived with prehistoric leaf-spring rear suspensions for so long, it's easy to see why settling for way less than the rest is the status quo. To their credit, Mopar fans have done an excellent job of managing their expectations, as making do with a suspension design straight out of the 17th century is no mean feat. Take heart: Since 2006, Reilly Motorsports (RMS) has been offering its Street-Lynx four-link suspension to classic Plymouth and Dodge owners (1962-76 A-Body, 1966-72 B-Body, 1970-74 E-Body). The Street-Lynx is a godsend to Mopars with true handling intentions; its lightweight dual-adjustable Viking coilover shocks and tunable link adjustments are just what the doctor ordered.

But simply having the ability to adjust coilover ride height, or to set your instant center by moving the lower control arm pickup point doesn't begin to tell the whole story. The RMS does do all this, but the immediate and dramatic improvement in ride quality and road-holding ability—due mostly to the drastic reduction in unsprung weight and the correction of geometry—will be appreciated the most.

To understand why the big improvement, let's look at the stock leaf spring. First, the leaf spring takes on the roles of both supporting the weight of the rear axle assembly, and locating the axle laterally. In order to do all that, it must be simple and massive. As the rear axle encounters bumps, all that iron must react. It's a slow, ungainly, thunderous dance that puts tire grip and ride comfort as a last priority. In smaller cars like Mustangs and Camaros, it's almost livable, but in larger cars like Mopar B- and E-Bodies, it'll rattle your fillings out.

Another problem on the performance side is rear roll steer. As a by-product of the leaf spring's arch design, when a leaf-spring car corners hard, the inboard spring shortens slightly as it arches more, while the outboard spring lengthens (as it flattens out). This actually rotates the axle and causes oversteer—it's unpredictable, it's nonlinear, and it's scary. If all this is happening while the tires are going over an uneven surface, the rear roll steer combines with the unsprung mass trying to react to the road, and you've got some real monkey motion going on. It should go without saying that you won't be able to add power in this situation.

For drag-only and street/strip cars, a leaf-spring rear creates problems with axlewrap on the launch pad. The more torque the engine puts out, the worse the problem gets. In this situation, the torque of leaving hard with a sticky tire causes the leaf spring to de-arch, or deflect into an "S" shape. As the leaf bends, it stores increasing energy, until that energy overcomes the grip and spins the tire. When it happens repeatedly in sympathetic resonance, it turns into wheelhop. Outside of the negative effect on launch traction, the pinion angle is distorted to the point where axle and U-joint breakage is common. The "fix" is often to put ever thicker, heavier leaves on, and/or add links to the already massive leaf spring's unsprung weight.

2/191. The installed RMS Street-Lynx suspension in our Valiant along with a new Strange S60 rear. The triangulated four-bar allows the location of the axle to happen independently from rearend support duties. Coilover shocks reduce unsprung weight and vastly improve suspension tuning. The upper control arms are adjustable, allowing pinion angle to be tuned, and the lower control arms have instant center adjustment as well.

3/192. The factory Mopar A-Body leaf springs locate the rear while supporting it and determining ride height and pinion angle. They also have to fight the forces of axle twist under braking and acceleration. Low-cost and unsophisticated, it was merely adequate for its day.

4/193. After stripping away the factory leaf springs, leaf-spring pockets, and stock rearend, Outlaw Motorsports power-washed the chassis and sprayed it down with a coat of Eastwood Extreme Chassis Black satin paint. The first step is to remove the factory bumpstops from the framerails. Some fuel and brake lines may need to be temporarily relocated during the installation.

5/194. You’ll need to strip away the paint where the crossbar needs to be welded to the framerails. (The crossbar is the attachment point for the upper control arms.) This involves mocking up the RMS crossbar on the frame, scribing marks around the edge, then grinding the frame to bare metal in the affected area.

6/195. Mount the crossbar to the framerails using two of the bumpstop mounting holes and the leftover bumpstop fasteners. These bolts do not provide structural integrity—they are only to hold and locate the bar during welding operations.

7/196. Use a MIG welder to stitch the perimeter of the crossbar mounting plates to the framerails, then fill the rosette welds in the holes provided in the RMS mounting plate. After cooling, we used some Eastwood Extreme Chassis Black to coat the weld areas.

8/197. The next operation is to preassemble the lower control arms to the control arm brackets. The brackets have four studs that bolt through the same holes used for the leaf-spring mounting pocket. If your Mopar had an inboard leaf-spring relocation kit, good news: The RMS control arms will bolt directly to those.

9/198. The factory six-banger rear has a 7¼-inch ring gear, just enough strength for a go-kart. We tossed it in favor of a built S60 rear from Strange Engineering. (The price of a rear axle is not included with the RMS suspension, so you’ll need to make necessary arrangements for something strong enough.)

10/199. The S60 has a husky 9.25-inch ring gear, making it the champ in the strength department. Ours also had a Detroit Truetrac LSD, 3.54 gears, 35-spline axles, Torino axlehousing ends, a 52.25-inch width for A-Body Mopars, A-Body leaf-spring perches, and Wilwood 11-inch Dynalite disc brakes. Note that use of a Dana 60 rear requires the exhaust exit ahead of the rear axle. If you prefer an over-axle exhaust, RMS recommends you use an 8¾-inch Chrysler rear, or a Ford 9-inch.

The RMS Street-Lynx rear suspension solves all of this and adds some cool extras, like being able to easily set your ride height for the most bitchin appearance. With the Street-Lynx, the coilovers and the four-links divide the job of the leaf spring into two tasks—the links locate the rear and correct the arc of the axle travel, they maintain pinion angle, and they offer adjustment for instant center. The coilovers, by contrast, support the full weight of the axle, while doing it at a fraction of the weight penalty of leaves. Here, RMS relies on a pair of U.S.-made billet aluminum Viking Warrior double-adjustable shocks, which have independent control of compression and rebound damping—all in a lightweight package that looks like jewelry. Relating to the overall weight issue, we obtained four-corner weights of our Valiant project car prior to any modifications. Once the project is complete, we'll weigh the car again to determine the overall balance and weight. We think our goal is reasonable: replace the Slant Six engine and stock driveline with an all-aluminum 657hp Indy-built Wedge, upgrade the 904 automatic with a built TCI 727, fortify our suspension with the RMS AlterKtion and Street-Lynx bits, and beef up the rear with a Strange S60. We expect we'll gain minimal weight, or even zero weight in the process, and the Street-Lynx will be a contributing factor.

Other cool Street Lynx fact: converting from leaves to a four-link provides an extra ¾ inch of tire clearance on the inside. With B- and E-Bodies, the lower bars are flush with the wheelhouse, allowing tire sizes as large as 28x12.5 without any other modification. There is even a pair of easy trailer tie-down holes in the axle mounting plates (which match the AlterKtion's front suspension tie-down loops in the front).

Here, we're following up last month's RMS AlterKtion front coilover suspension with the company's Street-Lynx four-link rear suspension. Work on our '68 Plymouth was once again performed by the knowledgeable Ron Aschtgen of Outlaw Motorsports in Riverside, California. Outlaw does nothing but muscle cars and race cars, and their skills run the range from engine building and chassis work, to sheetmetal, paint, and wiring. They are a complete one-stop shop with everything done in-house, so if you're considering having your work farmed out instead of doing it yourself, we recommend them highly. Outlaw got the Street-Lynx done in under four hours, not including stripping the old parts off, cleaning, and repainting the A-Body's chassis with Eastwood Extreme Chassis Black. The Street-Lynx job was significantly easier because there's no cutting involved and only a few easy welds need to be made.

Soon, Outlaw will help us tackle the installation of our front Wilwood brakes. We'll take a close look at those, and show you a cool piece from RMS that makes installing Wilwood's GM-based master cylinder into a Mopar a cinch!

To check out a video of the RMS Street-Lynx suspension installation at Outlaw Motorsports, go to the video page at PopularHotRodding.com and search for "Project Valiant: RMS Street-Lynx Rear Suspension Installation."

11/1910. The control arm axle plates get mounted to the axle spring perches with a pair of U-bolts on each side. The shock mounts face forward; the U-bolts should only be finger tight for now. If you’re on jackstands, this operation can be done with the rear under the car.

12/1911. The billet aluminum lower shock mounts are a work of art, and bolt up to the axle plates with included hardware. The ride height can also be adjusted by moving the lower shock mounts in the holes higher or lower on the axle plates. The shock studs for the coilovers are then attached to the billet aluminum shock mounts with antiseize to prevent galling. Tighten these fully.

13/1912. Next, Outlaw’s Ron Aschtgen mates the prepped Strange S60 axle to the Valiant with a transmission lift. Your jackstands and a floor jack will suffice nicely. The initial operation here is to attach the lower control arms to the axle plates with the supplied fasteners.

14/1913. The remainder of the Street-Lynx rear suspension installation is largely given over to locating the axle in the proper lateral plane, and fixing the position of the upper control arm tabs for welding on the axlehousing. Here, Aschtgen is measuring the end of the axle in relation to the framerail, and adjusting the axle so that the measurement is the same for both sides with the axle level to the chassis.

15/1914. With the upper control arms attached to the crossbar with the provided rod ends and tapered spacers, it’s possible to assemble the axle tabs on the control arms and find the ideal mounting location on the axletubes (there is a long and a short tab for each side). This should be done with the axle at approximate ride height, all while double-checking the axle ends against the framerails. Now’s the time to make sure there’s no binding between the tabs and the control arms. Once you’re satisfied, tack weld the tabs to the axle.

16/1915. Now that the tabs are tacked in place, you can lower the axle to make working room for your welder. Our new S60 axle provided a great surface to weld on, but if you’re using an older unit, make sure to grind away any paint or wipe off grease from the axletubes before you position and tack weld the tabs.

17/1916. Snug up the axle U-bolts to finally fix the axle in its place. You are just about ready for beer-thirty!

18/1917. All RMS AlterKtion and Street-Lynx suspensions are equipped with Viking Warrior double-adjustable shocks. These are some of the nicest dampers we’ve seen, and include a multitude of features, like dual 19-position damper adjustment for compression and rebound, lightweight construction, and ride-height adjustability. Each U.S.-built shock is tested independently on a shock dyno, serialized, and comes with a two-year warranty. Viking also warrants their springs to maintain within 2 percent of their spring rate for the life of the spring. RMS additionally states that if you need a different spring rate from the one initially supplied, they will swap them out for you at no cost.

19/1918. The final stage is bolting on the Viking Warrior coilovers—start by attaching the lower mounts to the shock studs on the axle plates (the compression and rebound knobs should face outward), then raise the rear until the upper spherical joint is even with the mounting holes in the crossbar.